A school bus carrying 33 6th-grade students slammed into a tractor trailer near the central Illinois community of Litchfield.

Authorities say the accident Monday afternoon, which injured 11 students, occurred as the bus was headed to Grantfork Upper Elementary School in Highland from a field trip to the state Capitol in Springfield. Police say the bus rear-ended the truck seven miles north of Litchfield on Interstate 55.

One student was airlifted to a hospital in St. Louis with serious injuries. The unidentified driver of the bus was also seriously injured. Authorities say the other injured students were taken to St. Francis Hospital in Litchfield or Hillsboro Area Hospital with non-life threatening injuries.

Illinois State Police and local authorities are investigating the accident, which shut southbound Interstate 55 for several hours.